Field of the Invention
The present invention relates to an optical modulator module, and particularly to an optical modulator module which includes a plurality of optical modulation units and in which a plurality of signal supply lines for supplying a modulation signal are provided in each of the optical modulation units.
Description of Related Art
Many optical modulators are utilized in the optical communication field and the optical measurement field.
In order to transmit high-speed/large-capacitance information, in regard to a modulation method of the optical modulator, a transmission format of multi-level modulation or multi-level modulation adopting polarization multiplexing, such as quadrature phase shift keying (QPSK) and dual polarization-quadrature phase shift keying (DP-QPSK) employing phase modulation is also diversified from intensity modulation (on-off keying) and the like in the related art.
As illustrated in FIG. 1, in an optical modulator module, a chip (modulation substrate) 1 including an optical modulation unit is built in a metal package case 2.
In order to input a light wave to an optical waveguide which is formed inside the chip or to output a light wave from the optical waveguide, an optical fiber 5 is introduced into the package case 2 from the outside.
In addition, in order to supply an electrical signal to control electrodes (modulation electrode, bias electrode, and the like) provided for modulating a light wave propagating through the optical waveguide inside the chip, an electrical signal is introduced into the package case 2 from an external signal source 4 via a connector terminal 3.
Recently, the number of optical modulation units embedded in one optical modulator module tends to increase.
For example, in accordance with the complicated modulation method such as the multi-level modulation, the number of optical modulation units increases.
Moreover, the optical modulation units are provided in a manner corresponding to each wavelength for multi-wavelength communication, processing is performed by causing a plurality of optical modulators to be embedded in parallel in order to cope with high speed communication, and so on. Consequently, the number of optical modulation units built in one package keeps on increasing.
A modulation signal supplied to the optical modulation unit is introduced into a package case from a signal source outside the optical modulator module via a connector terminal penetrating a wall surface of the package case.
Moreover, the connector terminal and the optical modulation unit are connected to each other through a signal supply line. A modulation signal introduced through the connector terminal propagates through the signal supply line and arrives at the optical modulation unit.
In the related art, in order to cause each of the optical modulation units to have the same time required for a modulation signal to propagate through the connector terminal to the optical modulation unit, a configuration in which the signal supply lines have the same electrical length is employed.
Specifically, in Japanese Laid-open Patent Publication No. 2004-185046, as illustrated in FIG. 2, a delay line 62 is provided in wiring electrodes (60, 61) which are the signal supply lines disposed on the modulation substrate 1 in which a plurality of optical modulation units 10 are embedded.
In FIG. 2 and other drawings for describing wirings, in order to simplify the drawings, only a signal electrode is illustrated, and a ground electrode is omitted.
In addition, in Japanese Laid-open Patent Publication No. 2014-89310, as illustrated in FIG. 3, a delay line 82 is provided in relay lines (80, 81) which are the signal supply lines disposed on a relay substrate 8 disposed between the modulation substrate 1 and connector terminals (30, 31).
In FIG. 3, the optical modulation units provided inside the modulation substrate 1 are illustrated as dotted line frames (a1, a2) in a simplified state.
The wiring electrodes supplying a modulation signal to the optical modulation units (a1, a2) are indicated with reference signs 63 and 64.
The connector terminals (30, 31) and the relay lines (80, 81) of the relay substrate 8 are electrically connected to each other through wires 7 such as gold wires.
The relay lines of the relay substrate 8 and the wiring electrodes of the modulation substrate 1 are also electrically connected to each other through wires or the like.
In a case where the number of optical modulation units disposed on one substrate is increased, or in a case where a plurality of modulation substrates (substrate provided with optical modulation units) are disposed in an arranged manner, the electrical length of the signal supply line from the connector terminal to each of the optical modulation units becomes longer.
Furthermore, in regard to modulation characteristics of each of the optical modulation units, when the electrical length of the signal supply line becomes longer, the modulation characteristics in a high-frequency band are likely to deteriorate.
That is, in regard to the conversion efficiency (E/O characteristics with respect to frequency f) of converting an electrical signal into an optical signal, FIG. 4 illustrates characteristics which vary depending on the electrical length of the signal supply line connected to each of the optical modulation units, as illustrated in graphs G1 to G4.
For example, the graphs G1 to G4 schematically illustrate the E/O characteristics in a case where signal supply wires having electrical lengths different from each other are connected to four optical modulation units. The graph G1 corresponds to a case where the electrical length of the signal supply wire connected to the optical modulation unit is the shortest. On the contrary, the graph G4 corresponds to a case where the electrical length of the signal supply wire connected to the optical modulation unit is the longest.
Generally, when the electrical length of the signal supply line becomes long, as illustrated in the graph G4, the E/O characteristics significantly deteriorates due to a high frequency.
Therefore, when the electrical lengths of other signal supply lines are adjusted to match the signal supply line having the longest electrical length, for example, the graphs G1 to G3 in FIG. 4 are operated closer to the graph G4, so that the modulation characteristics of each of the optical modulation units also significantly deteriorate.
As described above, in an optical modulator requiring a plurality of optical modulation units, deterioration of the modulation characteristics has become an important problem.
In addition, in order to perform high-speed/large-capacitance communication of 100 GHz or higher, a multi-level digital coherent method is utilized by means of multi-level phase modulation, polarization multiplexing, and the like.
In the digital coherent method, in regard to mistiming of a signal caused due to a difference between the electrical lengths, an adjustment can be performed through signal processing using a digital signal processor (DSP) without having deterioration of transmission characteristics, deterioration of the E/O characteristics or unevenness of the E/O characteristics between the signal supply lines caused due to the electrical length of the signal supply line is an extremely important problem.
Furthermore, since a band-widening configuration (high-performance configuration) and a highly integrated configuration of the optical modulator of 400 G/1 T are required, the problem of deterioration of the modulation characteristics becomes a more important object.